/stopsound @p says it stops all sounds for the player but does not stop entity.player.levelup}} when the player earns levels. {{/stopsound @p player does stop {{entity.player.levelup}} though.
MC-116819
/stopsound without a specified source does not stop the entity.player.levelup sound
Resolved
Cannot Reproduce
1
Can't reproduce in 1.11.2. At least with command blocks.
/playsound entity.player.levelup master @pgets muted just fine with/stopsound @p